Directly Attached Storage (DAS) systems typically include High Density Drive (HDD) attached memory storage devices that provide permanent memory and Solid State Device (SSD) memory storage devices that serve as cache memories. The SSDs provide relatively expensive memory with much faster read and write times in comparison to the HDDs. A relatively small amount of SSD cache can therefore be cost effectively provided as temporary cache memory for recently and frequently accessed data, sometimes referred to as “hot data.” Using an SSD cache memory avoids the read and write latency inherent in the HDDs for recently and frequently accessed data.
Tiered cache systems (e.g., “Cachecade”) have been developed to provide significantly larger amounts SSD memory than conventional single-tier SSD cache. The SSD memory is organized into multiple levels or tiers in which a relatively small primary SSD cache typically resides on a single physical device (typically a memory chip), while the secondary SSD cache includes an array of physical devices (typically several memory chips).
The individual physical devices in the secondary SSD cache array may be referred to as “arms” in the disclosure. Like an array of attached HDDs, the secondary SSD cache array may be configured with any of the Random Array of Independent Disks (RAID) protocols currently existing or developed in the future to improve security of the data stored in the SSD cache array.
In conventional RAID volumes for HDDs, single and double parity protocols are often used to provide parity based data reconstruction in the event of failure or corruption of one or more of the drives. Data mirroring is also popular in RAID systems for HDDs to protect against loss of entire drives or servers. The relatively high cost of SSD memory results in a desire to maximize the utilization of SSD memory in cache systems. Certain memory storage practices used in RAID systems or HDD attached drives may therefore be undesirable in RAID system for SSD cache arrays due to the higher premium placed on memory space in SSD cache systems. However, RAID systems specifically tailored to SSD cache arrays have not adequately addressed this need. There is, therefore, a continuing need for methods and systems for improving the utilization of SSD cache systems. More particularly, there is a need improved memory utilization in RAID systems specifically tailored for SSD cache arrays.